Regulating repeater systems



Nov. 17, l925 D. F. WHITING REGULATING REPEATER SYSTEMS Filed Feb. 15, 1921 2 Sheets-Sheet l /m/ew/ar.'

REGULATING REPEATER SYSTEMS Fileg Nov. 17 l925- F'lb. l5 1921 2 Sheets-Sheet 2 800Cycl I8 l "-9 I6 y 0 -m Cycles fer .seco/1d by JM/Mja p.......m..;1f.-; 192s.

UNITED STATES PATENTl OFFICE.

DONALD F.. WHITING, 0F NEW YORK, Y., ASSIGNOR T0 WESTERN ELECTRIC COM- PANY, INCORPORATED, 0F NEW YORK, N. Y., A CORPORATION 0F NEW YORK.

IREGIU'LATIII' G REPEATER SYSTEMS.

Application led February 15, 1921.

T0 all wzom t may concern.:

Be it known that I, DONALD F. VVHITING, a citizen of the United States, residing at` New York, in the county of Bronx, Sta-te of New York, have invented certain new and useful Improvements in Regulating Repeater Systems, of which the following is a. full, clear, concise, and exact description.

lhis invention relates to regulating and equalizing transmission in a circuit carrying currents of different frequencies, as for instance signaling frequencies and voice frequencies.

lVhcn the temperature, and consequently the resistance, of the conductors of a cable circuit varies the attenuation loss Varies in approximately direct proportion to the variation of the resistance. This variation is approximately constant throughout a wide range of frequencies in the majority of circuits. However, for certain types of lines, such for example as very long four-Wire cable circuits with very light loading, the variation of the transmission equivalent of the circuit for low frequencies is different from the variations for high frequencies, a given resistance change causing a smaller variation of loss for low frequencies than for high frequencies.

In the application of A. B. Clark, Serial No. 253,486, filed September 10, 1918, entitled'Means for and method of regulating transmission over electric circuits, on which Patent No. 1,438,219, dated December 12, 1922, was granted, it has been proposed to compensate for changes in the transmission equivalent of a circuit which are due to temperature. changes by adjusting the gain of a. regulating potentiometer associated with a repeater in the circuit, the adjustments being made automaticallyy in response to temperature changes, and consequent resistance changes, in a.. pilot Wire closely paralleling the circuit conductors and therefore subject to substantially the same tem- Serial No. 445,075.

perature variations as the circuit conductors. However, in that'sche-me a change in the adjustment of the potentiometer causes the same gain change for low frequency current as for high frequency current.

An object of the present invention is to compensate for changes in the transmission equivalent of a circuit which vary with fre.-

quency, and to compensate for such changes for widely different frequencies simultaneously.

It is also an object of the invention to automatically compensate for such transmission changes where the changes in the transmission equivalents of the circuit are due to temperature changes.

Another object is to` utilize a regulating repeater to cause not only a positive or negative regulating gain or compensating gain required/for the section of line which has its transmission regulated by the regulating repeater, but also a normal gain. This normal gain may be, for instance, of the order of the gain of one of the constant gain or non-regulating repeaters in the circuit in cases Where there are such repeaters in addition to regulating repeaters.

Another object is to render the compensating effect of the regulating repeaters independent of substantial changs in the impedance of certain apparatus in the regui.

lating repeaters. Y

A further object is to make it feasible to use exactly the same design of telegraph composite equipment and exactly the same 4design of telephone equalizing equipment with the regulating repeaters that is used with the non-regulating repeaters.

Other objects of the invention will be ap parent from the detailed description of the accompanying drawing.

In accordance with this invention conipensation for changes in the transmission equivalent of a circuit which are due to temperature variation and which vary with frequency is effected for Widely different frequencies simultaneously by producing compensating variations in the gains of one or more regulating repeaters in the circuit equal and opposite to the variations in transmission equivalents occurring in the circuit. These variations in the repeater gain are effected by changing the setting of a gain adjusting potentiometer automatically in response to temperature'changes and consequent resistance changes in a. pilot wire which is subjected to. the temperature changes to which the circuit to be regulated is subject. The gain adjusting potentiometer is so constructed that the gain change produced by any given change of setting of the potentiometer varies with frequency substantially in accordance with the variation with frequency of the change of equivalent due to the temperature change which caused the given change of .potentiometer setting.

Figure 1 of the accompanying drawing Shows diagrammatically a form 0f regulating repeater and associated circuits embodying features of the invention. Figure 2 shows curves indicating the series values of resistance and capacity which the steps of a potentiometer embodied in Figure 1 should include under certain conditions. Figure 3 shows the variations of the gain frequency curves of a repeater which are secured by means of the potentiometer of Figure 1. Figure 4 shows part of a system embodying both constant gain or non-regulating repeaters and regulating repeaters, indicating how the regulating repeaters may be utilized as normal gain repeaters also. Figure 5 shows a form of non-regulating repeater in connection with which the repeater of Figure 1 may be used to especial advantage.

Referring to Figure 1, 5 is intended to represent a transmission line Which tends to vary its transmission equivalent by different amounts at different frequencies for a given change in the temperature of the line conductors. The line 5 has connected therein a regulating repeater 7 comprising a two stage amplifier set, between the two stages 9 and 11 of which is located a gain g adjusting potentiometer 13, the setting, of

which may be varied to change the degree of amplification obtained from the amplifier set. An auto-transformer 14 is connected in the output circuit of the tube 9 and receives the space current of that tube. The potentiometer 13 'is fed from the auto-transformer. As usual, a. condenser 16 having a capacity of the order of a microfarad is interposed in the circuit between the first tube and the grid of the second tube. Moreover, one or more small capacities, such as that designated 19', which may be shunted by high resistance such as that designated 18,

mensen are interposed in the line between the-auto transformer and the potentiometer, to discriminate against and attenuate low frequencies in favor of higher frequencies, in order to cause the gain frequency curve of the repeater 7 to have a pronounced droop at low frequencies when the potentiometer is in its highest voltage setting. The reasons for causing the gain frequency curve for this setting of the potentiometer to have such a shape will vappear hereinafter.

A pilot Wire 12 is so associated With line 5, as for instance by incorporation in the same cable with line 5, that the pilotwire is subjected to whatever temperature variations may be imposed upon the line 5. The pilot wire is included in'one arm of a lVheatstone bridge automatic regulating arrangement 15 which changes the setting of a movable contact 17 of the potentiometer 13 in response to temperature changes and consequent resistance changes inthe pilot wire. This lVheatstone bridge automatic regulating arrangement may be for instance of the type disclosed in the patent of'A. B. Clark referred to above.y

If the impedance of the potentiometer 13 were merely resistance all points on the gain frequency curve C (Figure 3) of the regu lating repeater 7 would be raised or lowered substantially the same amount for any given change in the setting of the contact 17 due to a temperature change for instance,'and therefore since the change in the transmission equivalent of the line due to the temperature change varies with frequency the gain change effected by the change in the setting of the potentiometer could be the correct compensating gain change for only one frequency. 1

It has been found desirable to use a potentiometer of a type indicated in the drawing embodying in addition to resistance 21 distributed capacities 19 in order that correct compensationvmay tbe effected for widely differing frequencies simultaneously. Thus, using illustrative values andratios, if in a given line a given change of temperature produces a certain change in transmission for 800 cycle voice current and produces a change one-half as great for 135 cycle signal current, approximately correct compensation can be secured simultaneously for both of these currents. For example, from the curves of Figure 3, excepting curve E, which are 21 observed, gain frequency curves of the regulating repeater 7 corresponding respectively to 2l potentiometer settings and bands of line temperatures, it is seen that the 800 cycle point on curve B which is a curve for the highest voltage setting of the potentiometer 13 is separated from the 800 cycle point on curve C which is the curve for the middle voltage setting or average line temperature setting of the IVO 'i v:ranma potentiometer by a vertical distance representing a ten-mile change of gain whereas at 135 cycles these two curves` are separated by a vertical distance representing a. change of gain only about one-half t at great. Similarly the 800 cycle point on curve D, which is the curve for the lowest voltage setting of the potentiometer, lies below t 1e 800 cycle point of curve C a vertical distance representing a negative change of gain of ten miles,rwhereas the vertical distance from the 135 cycle point on the curve C to the 135 cycle point on curve D represents a negative change of gain only about one-half that great. The 800 cycle points on the 2l curves are uniformly spaced a distance representing a gain change of one mile. It is apparent from curve C that when the line is at its average temperature and the potentiometer is consequently at its normal or middle setting, the gain of the .regulating repeater 7 is about 26 miles. When the line temperature varies from its average, value sufliciently to cause the potentiometer contact 17 to be moved one step in either direction, the repeater gain frequency curve changes to that shown immedlately above the curve C or that shown immediately below the curve C, depending upon the direction at the temperature variation. Each y movement of one step on the potentiometer corresponds to a positive or negative gain change of one mile equivalent for 800 cycle current.v n

Figure 4 shows part of alme for repeating in one direction, and is intended to represent part of any such lme which tends to vary its transmission equivalent by different y amounts at different frequencies for a given change in the temperature of the line conductors, as for instance part ofv one side of a four-wire repeating system of the type shown in the patent to Van Kestern, 1,189,- 411, July 4, 1916, or of the type shown in the patent to Campbell, 1,352,736, September a maximum transmission change of 20 miles (that is plus and minus 10 miles) .imposed on it for 800 cycle current by the temperature changes` during any time period over which the regulating repeater is to automatically compensate for transmission changes. lVith any given line the value of the maximum transmission change fpr the whole line for 800 cycle current will, of course depend u n the equivalent length of the line and t e ran e of temperatures t'which a line assumes. herefore, with re latng repeaters having a maximum gain c ange of 20 miles (plus 10 and'minus 10 miles) for 800 cycle current, thel number of regulating repeaters used in the line will be approximately the maximum transmission change for the whole line dividedby the maximum gain change (20 miles) to be obtained from each regulating repeater. Each regulating re eater is preferably locatedat about the mid le of the regulating section'of line which it regulates; thus, in Figure 4, showing the line 5 provided with regulating repeaters 7 and provided with constant ain or non-regulating repeaters 23 inserted 1n the line to introduce transmission gains-in the ordinary way, the line has mid-section termination both as regards the regulating sections and as regards the repeating sections or sections of line between each two successive repeaters in the line. Preferably the normal gain of each regulating repeater is approximately equal to the gain of each constant gain or nonregulating repeater 23, the regulating repeaters and the non-regulating repeaters being all equally spaced and each regulating repeater functioning not only as a gain changing means for the regulating section of line which it controls, but also as a constant normal gain repeater for introducing a normal gain in a. repeating section of the line just as does each of the non-regulating repeaters. The regulating repeater serves substantially as an ordinarywrepeater plus a means for automatically changing the gain of a regulating section which in the line shown includes several repeating sections. Figure 4 slic'z'z's each of the pilotwires 13 as extending over the entire length of the regulating section of telephone line which is regulated by the regulating repeater actuated by the Wheatstone bridge automatic regulating device to which that pilot wire is connected. Obviously, each pilot wire might extend over only a part of the corresponding re lating section.

he non-regulating repeaters 23, Figure 4, are of the two-stage amplifier type as shown in Figure 5, and each is similar to the regulating repeater 7 except that retard coil or autotrausformer 24 of repeater 23 has a 1 to 1 ratio instead of the 2 to l ratio of autotransformer 14 and thatv therefore the input coil 25 of repeater 7 has twice the turns ratio of the corresponding input coil 55 of the repeater 23 and except that the apparatus connecting the two vacuum tubes 59 and 6l ofthe repeater 23 which correspond tothe two vacuum tubes 9 and 11 of the repeater 7 differs from the a paratus situated between the two tubes o the repeater 7 in that there is no attenuating element corresponding to the attenuating element 19', 18 and the autotransformer 14 of Figure 1, the first tube 59 being directly connected across the potentiometer 53, the condenser 16 and the grid battery 22 which correspond to the potentiometer 13, the condenser 16 and the grid battery of Figure 1, and that the potentiometer 53 corresponding to the potentiometer-13 is merely a resistance potentiometer, having no capacities corresponding to capacities 19 ot Figure 1. The primary winding of the input coil of the first stage 9 of the regulating repeater is so designed as to simulate the input impedance of the corresponding input coil 55 of the non-regulating repeater 23. Thus, the regulating and the non-regulating repeaters Will have similar electrical characteristics both in their input and in their output circuits. This condition is highly desirable, the most note- Worthy of its advantages being that exactly similar telegraph composite equipment and exactly similar telephone equalizing equipment may be used with the regulating repeaters and the non-regulating repeaters.

Curve E, Figure 3 is the gain frequency curve of the repeater 23. lit is noted that for 800 cycle current the normal gain of repeater 7 is the same as the gain of repeater 23 and that for 135 cycle signaling current the normal gain of repeater 7 is very nearly the same as that of repeater 23. rlihe tivo curves are approximately' coincident over the Whole range of .the most important of the voice frequencies. rlihe theoretically correct value of the gain or repeater 7 for 135 cycle current which corresponds to any given gain ,of repeater 7 ior800 cycle current is the value obtained by adding to the gain of repeater 23, for 135 cycle current, one-half of the dii'erence between the given gain of repeater 7 and the normal gain of repeater 7 tor800 cycles. llt is apparent from Figure 3 that the values of gain of repeater Z for 135 cycle current are all Within onel halt mile of the theoretically correct values.

rlhe impedan of the potentiometer 13 at about 300 cycles per second should preferably be not greater than about 150,000 ohms; for otherwise the capacity reactance of the leads and of the other elements of the poterie tiometer itself with respect to ground might be comparable to the impedance oi the potentiometer itself, so that the high impedance potentiometer Would be effectively shunted. However, it is desirable that the external impedance of the output circuit ot the tube 9 be high in comparison to the internal impedance of said output circuit, so that tubes may be emploved in the first stage of the repeaters which have anode-cathode impedances varying vvidely Without causing the compensation eected by the potentiometer to be disturbed` by such variations. rlhe use of the autotransformer 14C having its high voltage side presented to the first. tube of the repeater and its low voltage side feedin the potentiometer 13 presents a high im ance .to the tube and yet permits the use o a comparatively low impedance otentiometer Without undue reduction of t electromotive force received by the potentiometer. Thus, with a 150,000 ohm potentiometer and an autotransformer having a voltage ratio of 2 to 1, Which means an impedance ratio of 4 to 1, the anode-cathode impedance of the tube employed in the first sta e of the repeater may vary between the imits of 50,000 and 150,000 ohms Without unduly changing the amount of the compensating gain produced by the regulating repeater, and still the electromotive force across the 150,00 ohm potentiometer will be halt as great as the electromotive force across the entire Winding of the autotransformer the apparent impedance of which is 600,000 ohms looking from the tube. lf instead oi' using the autotransformer a 450,000 ohm resistance in series with the 150,000 ohm potentiometer were employed to bring the external impedance of the output circuit of the tube up to 600,000 ohms, then the electromotive force received by the 150,000 ohm potentiometer Would be only one fourth of the electromotive force across the anode and cathode of the tube. Moreover, the autotranstormer also serves as a retard coil in the E battery circuit of the tube. @i course, a transformer having separate primary and secondary windings may be substituted for the autotransformer, but the autotransormer is preferable in that its windings require less space.

The curves in Figure 2 indicate the series value oit resistance and capacity which the 21 steps of a i50,000 ohm potentiometer of the general type shown at 13 in Figure 1 should include in the case in which, as vvas assumed above, the regulating repeater is to adjust its gain by one mile steps for 800 cycle current over a range of 20 miles oi gain change and the transmission for 135 cycle signaling currents is to be varied by the regulating repeater at one-halt the rate at which the transmission of the 800 cycle voice currents is varied. These curves are for the ideal case in Which as many condensers are employed in the potentiometer as there are steps upon the potentiometer and in case fewer condensers are employed the proper values of resistance and capacity for each setting may be determined by inspection from these curves. The curves ot Figure 3 were secured With'a potentiometer having only live condensers. By Way of example the capacities of the live condensers are: .020 mf., .180 inf., .090 mf., .097 mf., and .084 mf., respectively; the capacity of the condenser 19 is .0051 mf.; the resistance 18 is 290,000 ohms; and the values of the 21 steps of resistance and capacity used for the 2l potentiometer settlngs, corresponding to the curves of Figure 3 are as follows:

Mile steps at 800 Capacity. Resistance. cycles/sec.

. Ml. Ohms. -10 020 13, 700 -9 0 2, 300

The condenser 19 which is shunted by resistance 18 discriminates against and attenuates low frequencies of the order of 135 cycles for instance, in favor of the high frequencies such as 800 cycle voice current. Therefore the curve B, Figure 3 has a pronounced droop at the low frequencies. Curve C is without such droop because the impedance of the capacity included in the part of the potentiometer between the contact 17 `and the lower end of the potentiometer is so much greater for the 135 cycle current than for the 800 cycle current that, notwithstanding the fact that all the potentiometer condensers and the condenser 19 tend to lower the value of 135 cycle current more than they tend to lower the value of 800 cycle current, the ratio of the 135 cycle voltage drop between contact 17 and the f lower end of the potentiometer to the 800 cycle voltage drop between these points is suiiiciently high to cause the repeater gain for 135 cycles to be almost as high as that for 800 cycles. Curve D has a pronounced rise at the low frequencies because with the contact 17 moved down near the lower end of the potentiometer the effect of the capacity included in the part of the potentiometer between these two points is similar to but greater than the effect` just noted, of the capacity between the point 17 and the lower end of the potentiometer when the potentiometer is at its mid-setting. The reason the effect is greater is that as the contact 17 is moved downward the ratio of the reactance between contact 17 and the lower end of the potentiometer to the resistance between these points increases.

dien the potentiometer is set to give curve C the effect upon the. gain of the repeater' of the condensers included in the part of the potentiometer lying between the low potential leads of the potentiometer a proximately neutralizes the effect upon t e repeater gain ofthe discrimination of the remaining potentiometer condenser and the condenser 19 against lo-w frequencies. The curve C therefore has approximately the shape of curve E at the average line temperature. Thus, the regulating repeater and the non-regulating repeater are interchangeable at the average line temperature.

When the potentiometer is set to give curve B, the discrimination of the condenser 19r shunted by the resistance 18 against low frequencies causes the gain of the repeater for 135 cycle current tobe approximately five miles less than it would be without such discrimination, notwithstanding the tendency of the condensers of the potentiometer to produce a contrary effect. Since the repeater regulating gain for 800 cycles with this potentiometer setting is 10miles the change of gain from the normal gain, or from the gain with the potentiometer at its mid-setting, to the gain with the potentiometer at its highest setting .is only about half as great for 135 cycle current as for 800 cycle current. Similarly, for the potentiometer settings corresponding to the curves between curves C and B the regulating gain of the repeater, or the gain change from the normal gain, is only about half as great for 135 cycle current as for 800 cycle current.

When the potentiometer is set to give' curve D, the effect of thecondenser included in' the part of the potentiometer between the low voltage leads of the potentiometer in tending to make the repeater gain greater for v135 cycles than vfor 800 cycles is so much greater than `the effect of the remaining condensers ofthe potentiometer and the condenser 19 in tending tol make the repeater gains less for 135 cycles than for 800 cycles that the difference in the ain from the normal is. about five miles ess for 135 cycles than for 800 cycles. Since the repeater regulating gain for 800 cycles with this potentiometer setting is '-10 miles the change of gain from the gain with the potentiometer at its mid-setting to the gain with the potentiometer at its lowest setting is only about half as great for 135 cycle current as for 800 cycle current. Similarly, for the potentiometer settings corresponding to the curves between curves C and D the regulating gain of the repeater, or the gain change from the normal gain, is only about half as `gre-at for 135 cycle current as for 80() cycle current.

Knowing the princicple upon which the potentiometer 13 and the attenuating element 19, 18 operate to cause the gain change produced at each change of setting of the potentiometer to be only a. fraction as great for 135 cycle current as for 800 cycle current, and to cause the gain frequency curve of repeater 7 to be substantially that of repeater 23 at the average line temperature, the data for plotting curves such as those of Figure 2 can be calculated when the resistance of the potentiometer and the resistance and the reactance of the autotransformer windin are given. The curves of Figure 2 are lllustrative of what may be plotted from such calculated data.

The broad invention of regulating and equaliziner 'transmission by varying repeater gain at different rates for different frequencies is the invention of A. B. Clark, and 1s claimed in an application in his name, Serial Number 353,487, led January 23, 1920, entitled Transmission equalization.

Although the principles of this invention have been set forth especially in connection with their application to the transmission of currents of audio frequencies, nevertheless, as stated above the values of frequencies mentioned herein are merely illustrative, and these principles may be applied in the transmission of currents of other frequencies, as for instance ultra-audio frequencies, without departing from the spirit of the invention or the scope of the claims.

What is claimed is:

1. The combination with an amplifier having input terminals and a source of electromotive force for said terminals of a potentiometer adjustable to different settings for causing changes in the electromotive force impressed on said terminals, said potent-iometer comprising means for causing said changes in electromotive force to vary with frequency for a given change in the potentiometer setting.

2. A repeater comprising a potentiometer adjustable to different settings for causing lchanges in the gain of said repeater, said potentiometer comprising means for' causing said changes of repeater gain to vary with frequency for a given change in the potentiometer setting.

3. A repeater comprising a single regulating device, consisting of a potentiometer, adjustable to different settings for causing changes in the gain of said repeater, said potentiometer comprising means for causing said changes to be less for low frequency than for high frequency for aI given change in the setting of said potentiometer.

4. The combination with an amplifier having a single input coil and having input terminals, of a potentiometer connected between said coil and said terminals for changing the electromotive force impressed on said terminals by greater amounts for high frequency than for low frequency for a given change in the potentiometer setting.

5. An amplifier comprising two amplifying elements in tandem relation and a potentiometer connected between said elements, said potentiometer comprising means for varying the degree of amplification effected by said amplifier by amounts varying' with frequency.

6. A non-regulating repeater and a gain changing repeater in tandem relation, and means for causing the gain frequency curve of the gain changing repeater to have a negative slope at low frequencies algebraically less than the slope of the gain frequency curve of the non-regulating repeater at the same low frequencies.

7. The combination with an amplifier having input terminals of a source of electromotive force for said terminals, and a potentiometer for varying the electromotive force impressed on said terminals, said potentiolneter comprising resistance and condensers connected in series between the output leads of said potentiometer.

8. A repeater comprising a gain changing potentiometer, said potentiometer c omprising a low capacity, high reactance condenser connected in series with a resistance.

9. A translating device having input terminals, a source of electromotive force of different frequencies therefor, means associated with said device for passing high frequencies from said source to said device more readily than low frequencies and a potentiometer connected between said source and said terminals, said potentiometer comprising means for producing a greater reduction in the high frequency voltage supplied to said terminals than in the low frequency voltage supplied to said terminals for a given change of setting of said potentiometer.

10. The combination with an amplifier having input terminals and a source of electromotive force for said terminals, of means connected between said source and said terminals for passing high frequencies more readily than low frequencies, and a potentiometer for varying the electromotive force impressed on said terminals, said potentiometer comprising reactance.

11. A repeater comprising a single means capable of adjustment to various settings for causing substantial changes of repeater gain both for signaling frequencies of the order of 135 cycles per second and for the upper frequencies of the voice range. said means comprising serially connected resistances and condensers.

12. The combination with a line subject to changes in its transmission equivalent which vary with frequency, of an adjusttable potentiometer comprising means for substantially completely compensating for such changes throughout a wide range of frequencies.

13. The combination with a line subject to changes in its transmission equivalent which vary with frequency, of a repeater therefor comprising a potentiometer adjustable tc different settings for causing repeater gain clian'gesso varying with frequency for each change in the potentiometer set-ting as to compensate for the variations in said changes in the transmission equivalent over the range of frequencies from signaling frequencies of the order of 135 cycles per second to the highest important voice frequencies.

14. The combination with a line subject to changes in its transmission equivalent which vary with frequency, of a repeater therefor comprising a potentiometer adjustable to different settings for causing repeater gain changes so varying with frequency for each change in the potentiometer setting as to compensate for the variations in said changes in the transmission equivalent over the range of frequencies from signaling frequencies of the order of 135 cycles per second to the highest important voice frequencies, and means associated-with said repeater for causing the gain frequency7 curve of said repeater to drop moderately at low frequencies with said potentiometer as set to give the greatest repeater gain.

15. The method of tansmitting.; voltage over a line which discriminates against the transmission of a voltage component of one frequency in favor of the transmission of a voltage component of another frequency, which comprises discriminating to a certain degree against the transmission of the component of said other frequency in favor of the transmission of the component of said one frequency and simultaneously favoring to a different degree the transmission of the component of said other frequency with regard tothe transmission of the. component of said one frequency and varying said ditferent degree.

16. A. line, a' repeater therefor comprising a potentiometer. said potentiometer comprising means for causing changes of repeater gain varyingwith frequency for a given change in the potentiometer sett-ing, and means for varying` the potention'ieter setting in accordance with impedance changes in said line.

1T. A line. a repeater therefor comprising a potentiometer for causing changes ot repeater `gain varying with frequenmv for a given change in the potentiometer setting. and .means for varying the potentiometer setting in accordance with impedance changes in saidline, said potentiometer com prising resistanees and reactances connected in series. i

18. A line, a. repeater therefor comprising a single regulating device. capable of adjustment to different settings, for causing changes of repeater gain smaller for low frequency than for high frequency for a given change of setting of said device, and means, including a pilot wire, for varying the setting of the potentiometer in accordance with temperature variations of said line.

19. A line subject to changes in the transmission equivalent which vary with frequency, a repeater therefor comprising a potentiometer -for causing changes of repeater gain varying with frequency for a given change in the potentiometer setting, and means for varying the potentiometer setting in accord ance with impedance changes in said line,

said potentiometer comprising ractance.

20. The combination with a line subject. to changes in its transmission equivalent which vary with frequency, of a potentiometer and operating means therefor for so controlling said line as--to compensate for said changes over the range of frequencies from signaling frequencies of the order of 135 cycles per second to the highest important voice frequencies.

21. The combination with a line subject to changes in its t `ansmission equivalent wh ich vary with frequency, of a potentiometer and operating means therefor for so controlling said line as to compensate for said changes for the range of frequencies from signaling frequencies of the order of'135 cycles per second to the highest important voice frequencies, means associated with said line for passing high frequencies more readily than low frequencies.

22. The combination with a line subject to changes of its transmission equivalent with temperature which vary with frequency, of a repeater therefor comprising a potentiometer for causing repeater gain changes to so vary with frequency for each change in the potentiometer setting as to compensate for the. variations in said changes in the transmission equivalent over a range ,of frequencies from signaliig frequencies of the order of 135 cycles pd' second to the highest important voice frequencies, and

means including a pilot wire subjected to the temperature changes to which said line 1s subject, for automatically setting said potentiometer in response to said temperature l changes.

23. A line and a potentiometer, part of said potentiometer being connected in serial relation to said line. and another part. thereot' including capacities and a resistance connected in series therebetween, and said other part forming a current path from one, side of said `line to the other side through said 'apacities and resistance in series.

Q4. A potentiometer comprising a current path including resistance. and capacit-y connected in series, potentiometer output leads for connection to said path, and means for adjustably connect-ing said leads to said path in such manner as to vary the ratio of rcsistance to capacity included between the CTI points of connection of said leads to said path.

25. A line, and a gain changing repeater' therein, said repeater comprising means for regulating the transmission equivalent of said line automatically in response to changes in conditions affecting saidequivalent, said repeater having a normal gain greater than zero.

26. A line, and a gain changing repeater therein, said repeater comprising means for regulating and equalizing thc transmission equivalents of said line for widely different frequencies automatically in response to changes in conditions affecting said equivalents, said repeater having a normal gain greater than zero. Y.

27. A line, a constant gain non-regulating repeater therefor, and a gain changing regulating repeater for said line, the gain frequency curves of said repeaters being in substantial coincidence over the range of frequencies from signaling frequencies of the order of 185 cycles per second to the highest important voice frequencies, When said regulating repeater is set for its normal gain.

28. A line, and means for regulating the transmission equivalent of said line automatically in response to temperature variations to which said line is subjected, said means including a gain changing regulating repeater and means for causing the repeater gain to be greater than zero when the line is at the average temperature.

29. A line subject to changes in its transmission equivalent which vary with frequency, means responsive to temperature variations of said line for compensating for said changes of equivalent for widely different frequencies simultaneously, said means comprising a gain changing regulating repeater and means for causing the repeater gain to be greater than zero when said line is at its average temperature.

30. A line, and repeaters therefor including gain changing regulating repeaters having means operative automatically in response to changes in 'conditions affecting the transmission equivalent of the line 'to regulate the transmission equivalent, and constant gain non-regulating repeaters, all of said repeaters being substantially equally electrically spaced.

3l. A line, and repeaters therefor, including gain changing regulating repeaters having means operative automatically in response to changes in conditions affecting the transmission equivalent of the line to regulate the transmission equivalent, and constant gain non-regulating repeaters, said regulating repeaters having a normal gain of the order of the gain of said non-regulating repeaters. 32. A line, repeaters therefor, including gain changing regulating repeaters having means operative automatically in response to changesI in conditions affecting the transmission equivalent of the line to regulate the transmission equivalent, and constant gain non-regulating repeaters, said regulatmg repeaters having a normal gain substantially greater than zero.

33. A line, and repeaters therefor, including gain changing regulating repeaters having means operative automatically in response to changes in conditions affecting the transmission equivalent of the line to regulate the transmission equivalent, and constant gain non-regulating repeaters, all of said repeaters being substantially equally electrically spaced and said regulating repeaters having a normal gain of the order of the gain of said non-regulating repeaters.

34. A line, a constant gain non-regulating repeater for said line and means for regulating the transmission equivalent of said line in response to temperature variations of said line, said means comprising a gain changing regulating repeater and means for causing the gain of said regulating repeater to be of the order of the gain of said nonregulating repeater when said line is at its average temperature.

35. A line, constant gain non-regulating repeaters for said line, and mea-ns for regulating the transmission equivalent of said line in response to temperature variations of said line, said means comprising gain changing regulating repeaters and means for causing the gains of said regulating repeaters to be of the order of the gains of said non-regulating repeaters When said line is at its average temperature, and all of said repeaters being substantially equally spaced.

36. The method o f controlling the transmission equivalent of a line provided with a constant gain non-regulating repeater and a gain changing regulating repeater which comprises adjusting the gain of said regulating repeater to values above and below the value of the gain of said non-regulating repeater.

3 7. The method of controlling the transmission equivalent of a line provided With a constant gain non-regulating repeater and a gain changing regulating repater which comprises adjusting the gain of said regulating repeater to values above and below the value of the gain of said non-regulating repeater, over the frequency range from frequencies of the order of 135 cycles per second to the important voice frequencies.

38. The method of controlling the transmission equivalent of a line provided with a constant gain non-regulating repeater and an adjustable gain regulating repeater which comprises setting said regulating repeater for a normal gain of the order of the gain of said non-regulating repeater, and

increasing and decreasing the gain of said regulating repeater above and below normal.

39. The method of controlling the transmission equivalent vof a line provided with a constant gain non-regulating repeater and an adjustable gain re ulating repeater which comprises setting said regulating repeater for a normal gain of the order of the gain of said non-regulating repeater, and increasing and decreasing the ain of said regulating repeater Aabove angl below normal by amounts varying with frequency.

40. The methodof controlling the transmission equivalent of a line provided with a constant gain non-regulating repeater and an adjustable gain regulating repeater which comprises setting said regulatin repeater for a normal gain of the ordero said nonregulating repeater, and varying tlievgain of said regulatingrepeater.

41. The method of operating an adjustable gain changing repeater to compensate for effects of temperature changes on the line which comprises setting said repeater to give a normal gain greater than zero when the line is at the average lof the temperatures within the range for which compensation is to be effected, and increasing and decreasing the gain above and below normal when the line tempera-ture changes from lsaid average.

42. A line, constant gain non-regulating repeaters and gain'changin regulating repeaters for said line, the e ectrical charactei-'istics of the input and the output circuits of said regulating repeaters being similar Ato the electrical characteristics of the input andthe output circuits, respectively, of said non-regulating repeaters.

43. A line, constant gain non-regulating repeaters and gain changing regulating repeaters for said line, the e ectrical characteristics of the input and the output circuits of said regulating repeaters being similar to the electrical characteristics of the input and p the output circuits, respectively of said nonregulating repeaters, said regulating repeaters having a normal gain of the order of the.

' 45. A line, constant gain non-regulating repeaters therefor and regulating repeaters for saidl line comprising two amplifiers, a potentiometer fed from said transformer and feeding the other of said amplifiers, said potentiometer comprising distributed capacities and the input and the output impedance of the regulating repeaters being similar to the input and the output impedance, respectively, of the non-regulating? repeaters.

' 46. The method of connecting twov amplifying elements in tandem relation which comprises so coupling the output terminals of the first element to the input Vterminals of the second element that the ratio of the external impedance of the output circuit ofv the first element to the external impedance of the in ut circuit o f the second. element is reater t an the ratiofof the eleetromotive orces across said impedances, respectively.

47. A repeater comprising two repeating elements in tandem and a transformer con.-

necting the outputterminals of one ofl said elements to supply electromotive force for the input terminals of the other element, said transformer having an im edance ratio greater than unity'and having its high impedance side presented. to said one element.

48. A repeater comprising an amplifier, a transformer fed from said amplifier, a potentiometer fed from vsaid transformer, and an amplifier fed from said potentiometer,v

said transformer having an .impedance ratio greater than unity and havin its high imv pedanceside presented to the l t mentioned amplifier.

49. A repeater comprising an amplifier, a transformer fed from said amplifier, a potentiometer fed from said transformer, and an amplifier fed from said potentiometer, said transformer having an impedance ratio greater than unity andl having its high imtioned amplifier, and said' potentiometer comprising serially conneeted'resistance and capacity.

50. A and a potentiometer for varying the gain of said repeater, said potentiometer comprising means adjustable to different, settings fori causing the Vrepeater gain to be greater' for edance side presented tothe. first men-v repeater comprising an amplifier i low frequenciesv than for high frequencies in onesetting of said potentiometer, and

for causing the repeater gain to 'be less'for said low frequencies than for said high frequencies in another setting of saidv potentiometer. f

In witness whereof, I hereunto subscribe my name this 7th day of Feby., D., 1921.

DoNALD F. WRITING. 

